Lore

Firekeepers are the sworn caretakers of certain Bonfires found in Lordran. Their presence maintains the flame of a Bonfire, and should they be slain, the Bonfire they take care of will die. Whether by chance or design, all Fire Keepers are female (presumably even the Stone Dragon, because of the nest).

Fire Keepers seem to have some form of disability: Anastacia is mute and maimed; Quelaag's Sister is physically warped by the Bed of Chaos and sick from the pus she sucked from the residents of Blighttown; and the Darkmoon Knightess is said to be concealing hideous swarms of humanity under her brass armor.

Although the Bonfire in front of the Stone Dragon is lit upon the player's arrival, it is safe to ignore the possibility that it may be a Firekeeper due to the fact that Ash Lake is a metaphorical place.

Theories

Title of theory.

Sources

Fire Keeper Soul (Generic)Soul of a long-lost Fire Keeper.
Each Fire Keeper is a corporeal manifestation of her bonfire, and a draw for the humanity which is offered to her.
Her soul is gnawed by infinite humanity, and can boost the power of precious Estus Flasks.

Fire Keeper Soul (Anastacia of Astora)Soul of the Ash Maiden, Fire Keeper of Firelink Shrine.
A Fire Keeper's soul is a draw for humanity, and held within their bosoms, below just a thin layer of skin, are swarms of humanity that writhe and squirm.
Was the Ash Maiden locked in this dark prison for some transgression, or by her own will?

Fire Keeper Soul (Lady of the Darkling)Soul of the Darkmoon Knightess, Fire Keeper of Anor Londo.
A Fire Keeper's soul is a draw for humanity, and held within their bosoms, below just a thin layer of skin, are swarms of humanity that writhe and squirm.
Her brass armor serves to disguise this ghastly form.

Fire Keeper Soul (Quelaag's Sister)Soul of a Daughter of Chaos, Fire Keeper of Quelaag's Domain.
A Fire Keeper's soul is a draw for humanity, and held within their bosoms, below just a thin layer of skin, are swarms of humanity that writhe and squirm.
To her, the countless eggs which appeared were cradles for each tiny humanity.

Estus FlaskThe Undead treasure these dull green flasks.
The Estus Flasks are linked to the Fire Keepers. The Dark Tales also make reference:
An emerald flask, from the Keeper's soul.
She lives to protect the flame, And dies to protect it further.

Dingy SetWorn by the Fire Keeper at Firelink Shrine.
It is thought to have once been the white hood of a maiden, but smoke and ashes from bonfires darkened it over the years.

Blood-Stained SkirtWorn by the Fire Keeper at Firelink Shrine.It is thought to have once been the white skirt of a maiden, but its true origin is lost in patches of blood.
Perhaps its former wearer was maimed to prevent escape?